The present invention relates to an ultrasonic motor and an electronic apparatus having the ultrasonic motor, and particularly to a method of controlling an ultrasonic motor in which a self-excited oscillation circuit constituted by using the ultrasonic motor per se is operated as a drive circuit, and to an electronic apparatus using the ultrasonic motor.
An ultrasonic motor begins to be used in various electronic apparatus since the ultrasonic motor is provided with excellent features of being small-sized, having high torque and high positioning resolution, and having hold force when electricity is not conducted. The drive circuit of an ultrasonic motor generally needs a frequency tracking circuit for tracking resonance frequency of the ultrasonic motor which is varied by temperature or external load to thereby pose a problem of bringing about complicated formation of the drive circuit or large-sized formation or high cost formation of a circuit mounting unit thereof. Hence, in recent years, there has been an example in which simplification of a drive circuit is achieved by using a self-excited oscillation circuit constituted by using an ultrasonic motor per se and an amplifying circuit.
However, when a drive circuit is constituted by using a self-excited oscillation circuit, there is a drawback that a time period is taken from when the drive circuit is brought into a drivable (ON) state until oscillation is started and an amplitude of oscillation increases and grows to a signal necessary for driving the ultrasonic motor.
Although when an ultrasonic motor is used for controlling position or speed, it is necessary to operate the ultrasonic motor intermittently by a very small amount by providing an intermittent signal to the ultrasonic motor or to make speed variable, when such a control is carried out by using the self-excited oscillation circuit in the drive circuit, there poses a problem in which a drive signal actually produced by the self-excited oscillation circuit cannot follow intermittent commands of driving/stopping the drive circuit, the ultrasonic motor cannot be started or operation thereof becomes unstable by causing abnormal oscillation.
Hence, according to the invention, a command signal for controlling a state of a self-excited oscillation circuit constituting a drive circuit is set in a range capable of stably driving an ultrasonic motor in an aimed state.
That is, according to an aspect of the invention, in an ultrasonic motor apparatus constituting a self-excited oscillation circuit by a vibrating body having a piezoelectric element and an amplifier circuit for oscillating the vibrating body in a predetermined natural mode and operating a moving body in contact with the vibrating body, the self-excited oscillation circuit can be controlled to be brought into a drive state or a stop state in accordance with a drive command signal or a stop command signal from a control signal generating circuit, and the control signal generating circuit is set such that a length TD of the drive command signal is made to be longer than a time period t1 from when the drive command signal is applied to the self-excited oscillation circuit until the self-excited circuit starts oscillating.
In this case, particularly, the length of the time period t1 is made to be a time period t2 until the self-excited oscillation circuit starts oscillating by a frequency component used for driving the moving body.
Further, in accordance with another embodiment, the length TD of the drive command signal is made longer than a time period t3 until an amplitude of oscillation of the self-excited oscillation circuit grows and reaches an amplitude value capable of operating the moving body, or longer than a time period t4 until the amplitude of the oscillation of the self-excited oscillation circuit grows and is saturated to a specific amplitude value. Alternatively, the length TD is set to fall in a range between the time period t3 and time period t4.
Further, by setting the control signal generating circuit such that a length TS of the stop command signal becomes shorter than a time period t9 from when the stop command signal is inputted to the self-excited oscillation circuit until the oscillation is stopped, a state of the moving body is controlled without stopping the oscillation.
When the ultrasonic motor apparatus is driven by alternately inputting the drive command signal and the stop command signal to the self-excited oscillation circuit, the time period TD and the time period TS are set such that an amount of changing a voltage value of an input unit of the amplifier circuit during a time period for applying the drive command signal in the time period TD, becomes larger than an amount of changing the voltage value of the input unit of the amplifier circuit during a time period for applying the stop command signal in the time period TS.
Further, in making variable the speed of the ultrasonic motor, a frequency 1/(TD+TS) of a control command signal constituted by the time period TD of the drive command signal and the time period TS of the stop command signal, is set to be higher than a frequency of the predetermined natural mode.
Further, by switching an oscillation state of operating the moving body and an oscillation state for making the moving body unable to operate by making a number of amplifier circuits brought into an active state among a plurality of the amplifier circuits, periodically variable, the moving body is driven or stopped or a speed thereof is changed without stopping the self-excited oscillation circuit to thereby produce stable oscillation.
Further, by mounting the above-described ultrasonic motor to an electronic apparatus, controllability of the electronic apparatus is promoted and small-sized and low power consumption can be achieved.